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A wave of bomb threats emailed Thursday to hundreds of schools, businesses and government buildings across the U.S. triggered searches, evacuations and fear — but there were no signs of explosives, and authorities said the scare appeared to be a crude extortion attempt.

Law enforcement agencies across the country dismissed the threats, saying they were meant to cause disruption and compel recipients into sending money and were not considered credible.

Infinity Ward, a software company in Woodland Hills, was said to be one of the many places where an extortion email with a bomb threat was sent on Dec. 13, 2018. The firm's campus is pictured on that day. (Credit: KTLA)

In Southern California, authorities in Ventura and Riverside Counties said no credible threat had been found. Meanwhile, sheriff's officials in Los Angeles County also said no threats had been substantiated but they were continuing to investigate.

Some of the emails had the subject line: "Think Twice." They were sent from a spoofed email address. The sender claimed to have had an associate plant a small bomb in the recipient's building and that the only way to stop him from setting it off was by making an online payment of $20,000 in Bitcoin.

An elementary school and hotel in Santa Barbara County were among those targeted with the extortion email, according to the Santa Barbara County Sheriff's Office. Both threats are believed to be part of the same hoax.

"The threat was quickly determined to be part of a nationwide hoax and not credible," sheriff's officials wrote. "The FBI is investigating the threats, which appear to be originating from outside the United States."

The New York City Police Department's counterterrorism unit tweeted that it was also investigating the spate of threats.

"We are currently monitoring multiple bomb threats that have been sent electronically to various locations throughout the city," the unit tweeted. "These threats are also being reported to other locations nationwide & are NOT considered credible at this time."

Other law enforcement agencies also dismissed the threats, which were written in a choppy style reminiscent of the Nigerian prince email scam.

The Palm Beach County, Florida, sheriff's office and the Boise, Idaho, police said they had no reason to believe that threats made to locations in those areas were credible. One of the emails wound up in a spam filter, Boise Police Chief William Bones said.

The FBI said it is assisting law enforcement agencies that are dealing with the threats.

"As always, we encourage the public to remain vigilant and to promptly report suspicious activities which could represent a threat to public safety," the FBI said in a statement.

Across the country, some schools closed early and others were evacuated or placed on lockdown because of the hoax. Authorities said a threat emailed to a school in Troy, Missouri, about 55 miles (88 kilometers) northeast of St. Louis, was sent from Russia.

The bomb threats also prompted evacuations at city hall in Aurora, Illinois, the offices of the News & Observer in Raleigh, North Carolina, a suburban Atlanta courthouse and businesses in Detroit.

"Organizations nationwide, both public and private, have reported receiving emailed bomb threats today," Michigan State Police spokeswoman Shannon Banner said. "They are not targeted toward any one specific sector."

Penn State University notified students via a text alert about threats to a half-dozen buildings and an airport on its main campus in State College, Pennsylvania. In an update, the school said the threat appeared to be part of a "national hoax."

Officials at Columbine High School in Colorado were dealing Thursday with a bomb threat of a different sort. Students were being kept inside for the rest of the school day after someone called in a bomb threat against the school.

The Jefferson County, Colorado, Sheriff's Office said the caller claimed to have placed explosive devices in the school and to be hiding outside with a gun.

There is nothing to validate the threat was found at Columbine, where 12 students and a teacher were killed by two students in 1999, according to Sheriff's spokesman Mike Taplin.

Two dozen other Colorado schools were also temporarily placed on lockout, meaning their doors were locked but classes continued normally, as the threat was investigated.

Threats in Chicago are part of a similar pattern being made nationwide. #ChicagoPolice are working with federal partners on the investigation, and at this time there is no elevated threat level for the city of Chicago pic.twitter.com/9Bo8K5tADd